This invention is related generally to the measurement of temperature by optical techniques, and specifically to the measurement of very hot environments, such as those in excess of 450 degrees centigrade, using optical temperature sensors attached to optical fibers.
As an alternative to the use of metallic temperature sensors, such as thermocouples or thermistors, in environments not suitable for their use, optical fiber temperature sensors are being commercialized in various forms. Such sensors are typically formed from a temperature sensitive optical material or structure carried at end of an optical fiber transmission medium. Located at the other end of the fiber medium is an electro-optical instrument which directs radiation to the optical sensor, and which receives back and detects returning radiation which has been modified in a way relating to the temperature of the sensor's environment. The temperature dependent characteristics of the returning radiation are then measured by the instrument and converted to the reading of the temperature of the optical sensor.
Many different types of such optical temperature sensors have been proposed in the literature, some of which have found commercial application. One class of sensors simply reflects or absorbs light as a function of its temperature, so that the proportion of the light intensity sent by the instrument which is then returned to the instrument is proportional to temperature. Examples are given in U.S. Pat. Nos. 4,016,761 - Rozell (1977) and 4,136,566 - Christensen (1979). Another type of sensor, exemplified by U.S. Pat. No. 4,140,393 - Cetas (1979), uses a birefringent crystal which alters the polarization of incident light as a function of its temperature. Yet another type of sensor is an optical etalon, as in U.S. Pat. No. 4,678,904 - Saaski et al. (1987).
The most development has gone into sensors made of luminescent materials. The instrument sends optical radiation in one wavelength range along the optical fiber to the sensor in order to excite the sensor material to luminesce in another wavelength range. A characteristic of the returned luminescent radiation that is temperature dependent is then detected as a measure of the sensor's temperature. In one implementation, a ratio of the intensity of the luminescence in different wavelength bands is taken, the ratio being proportional to the sensor's temperature. This implementation and use of luminescent fiber optic sensors generally are described in U.S. Pat. Nos. 4,075,493 - Wickersheim (1978), 4,215,275 - Wickersheim (1980), 4,448,547 - Wickersheim (1984) and 4,560,286 - Wickersheim (1985). In another implementation, the luminescent sensor is repetitively pulsed with exciting radiation and then the rate of decay of the luminescent intensity between pulses is determined as a measure of temperature. This is the implementation that is being most widely commercialized, examples of which are given in U.S. Pat. Nos. Re. 31,832 - Samulski (1985) and 4,652,143 - Wickersheim et al. (1987).
There are many advantages of fiberoptic temperature measuring systems but they do have a limitation as to the maximum temperature at which they can operate. For example, commercial luminescent sensors are presently only available for measuring temperatures up to 450 degrees centigrade. With selected materials, higher temperatures could be measured, but only over a limited range. However, there are many applications where point temperature measurements substantially above 450 degrees centigrade are desirable or necessary. Presently available fiberoptic sensors, generally, cannot measure such temperatures for any one of several reasons. One such reason is that the temperature dependent characteristic of the sensor material may become difficult or impossible to measure at very high temperatures. Another reason is that most optical fibers and many optical sensor materials cannot operate at high temperatures because of undesirable sensor or fiber changes which occur in such a severe thermal environment.
One attempt at overcoming these difficulties is to substitute a small blackbody for the optical sensor and communicate its infrared emission along a crystalline rod or optical fiber that transmits reasonable amounts of infrared radiation and can withstand the high temperatures involved. At a distance from the hot environment or object being measured, where the temperature is substantially lower, the infrared-transmitting rod or fiber is connected to a standard low temperature optical fiber to communicate the infrared radiation from the blackbody to a detecting station. The intensity of the detected infrared radiation is proportional to the temperature of the blackbody sensor. Examples of such a sensor and system are given in U.S. Pat. Nos. 4,576,486 - Dils (1986) and 4,679,934 - Ganguly et al. (1987), and in a paper by Holmes, "Fiber Optic Probe for Thermal Profiling of Liquids During Crystal Growth," Rev. Sci. Instrum. 50(5), May 1979, pages 662-3.
These systems suffer from a limitation of conventional optical fibers with regard to their poor transmission of infrared radiation. Electromagnetic radiation in the infrared region of the spectrum is significantly attenuated by such fibers. Only the shortest wavelength (near-visible) infrared radiation from such blackbody sensors can be transmitted via such optical fibers. Until better infrared transmitting fibers are developed for use in such applications at a reasonable cost and with acceptable thermal and mechanical properties, this will remain a significant limitation of such systems.
Therefore, it is a primary object of the present invention to provide a high temperature measuring system utilizing an infrared radiating sensor and conventional optical fibers that overcome these disadvantages.